When only one fixture runs weak while the rest of the house is fine, the cause is local to that fixture, not the main supply. The usual culprits are a scaled aerator or showerhead, a partly closed supply stop valve, a kinked supply line, or a clogged cartridge. Clean or replace the part to fix it.
How to tell a single-fixture problem from whole-house low pressure
Start by ruling out the bigger issue. Turn on the suspect fixture, then test two or three others in different parts of the home. If the rest deliver a strong stream, your supply pressure is fine and the trouble is isolated. If several fixtures run weak at once, you are looking at a whole-house problem instead, which points to the PRV, the main shutoff, or corroded pipe, covered on our low-water-pressure-whole-house page rather than here.
Next, separate hot from cold. With the fixture running, open the hot side alone, then the cold side alone. If both are weak, the restriction is downstream of where the two lines meet, meaning the aerator, the cartridge, or the fixture body. If only one temperature is weak, the clue points upstream of the mixing point on that one line. A cold-only weak side usually means a clogged cold supply stop or supply line. A hot-only weak side often traces back to the water heater side: a partly closed hot stop, sediment in the heater, or a failing flex line on the hot run. That hot-versus-cold split is the fastest way to narrow the search before you take anything apart.
Normal home pressure runs 40 to 80 psi, and EPA WaterSense notes fixtures operate best at 45 to 60 psi. If a gauge at a hose bib reads in that range with all water off, your supply is healthy and the single weak fixture confirms a local clog or valve. A simple screw-on pressure gauge from a hardware store threads onto an outdoor hose bib in seconds, and reading it before you take anything apart saves you from chasing a fixture problem that is really a supply problem, or the reverse.
One more quick check separates a clog from a valve. Fill a measuring cup at the weak fixture and time it. If the stream barely fills the cup while the same test at another sink fills it fast, the restriction is real and physical, not your imagination. That timed flow also gives you a before-and-after, so you can confirm a cleaning actually worked instead of guessing.
The most common local causes, from likely to rare
Work through these in order, since the first two are the easiest to check and the most common in Phoenix. Each one restricts flow to that single fixture without touching the rest of the home, which is why the symptom stays isolated.
- Clogged aerator or showerhead. The screen on a faucet tip or the spray face of a showerhead catches the most scale because the openings are tiny. Phoenix water is hard, so calcium and magnesium build up fast here.
- Partly closed supply stop valve. The small oval or football-shaped knobs under the sink or behind the toilet are stop valves. If one was bumped or left half-closed after a repair, that fixture alone runs weak. Open it fully (counterclockwise).
- Kinked or corroded supply line. The flexible braided or ribbed line between the stop valve and the fixture can kink behind the cabinet or corrode internally, choking flow to that fixture only.
- Clogged cartridge. Single-handle faucets and shower valves use a cartridge that mixes hot and cold. Scale and grit lodge inside it and cut flow. This often shows up as both hot and cold being weak at one fixture.
- Clogged shower flow restrictor. Showerheads contain a small flow restrictor disc that meets federal flow limits. Sediment packs into its narrow center hole and strangles the spray even when the rest of the head is clean.
Why Phoenix hard water is usually the reason
Scale is the thread running through most of these causes, and Phoenix has an unusual amount of it. The USGS classifies water as very hard above 180 mg/L of calcium carbonate. City of Phoenix water quality reports put total hardness at roughly 170 to 284 mg/L, about 10 to 17 grains per gallon, which sits at the top of "hard" and into "very hard." The USGS puts it plainly: "Water is a great solvent for calcium and magnesium, so if the minerals are present in the soil around a water-supply well, hard water may be delivered to homes."
Those dissolved minerals drop out of solution and harden on any surface water touches, and they collect first at the smallest openings, which is exactly where flow gets choked. That is why aerators, showerhead faces, and cartridges clog years sooner here than in soft-water regions. Heat speeds the process, so the hot side of a fixture often scales faster than the cold, which is one reason a hot-only weak stream is common in older Phoenix homes.
A whole-house water softener slows this buildup across every fixture at once, but it is not required to fix the problem in front of you. Cleaning the affected part restores flow right away, and in hard water you can expect to repeat that cleaning every year or two on the fixtures you use most. Knowing scale is the cause also tells you what not to do: there is no chemical you pour down a faucet to clear it, since the clog sits in the screen and cartridge, not the drain.
How to test and fix each cause
Move from the cheapest check to the most involved.
Aerator. Unscrew the aerator from the faucet tip by hand or with tape-wrapped pliers. Turn the faucet on with the aerator off. If flow is strong, the aerator was the problem. Soak it in white vinegar for 30 to 60 minutes to dissolve the scale, scrub the screen with an old toothbrush, rinse, and reinstall. Replace it if it is corroded. A standard bathroom faucet aerator runs at 1.5 gallons per minute at 60 psi under WaterSense, so a healthy one should refill a cup quickly.
Showerhead and flow restrictor. Unscrew the head and soak it in white vinegar for an hour, or tie a bag of vinegar over it in place overnight. Clear the spray holes and the small flow restrictor disc inside with a toothpick. Rinse and reattach.
Supply stop valve. Reach under the sink or behind the toilet and turn the stop valve fully counterclockwise. If it was partly closed, flow returns at once. If the valve feels gritty or will not open fully, it may need replacement.
Supply line. Inspect the flex line for a sharp kink behind the cabinet and straighten it. If it is old or you suspect internal corrosion, shut the stop, disconnect the line, and check whether flow at the stop itself is strong. Weak flow at the open stop points upstream; strong flow there points to the line or fixture.
Cartridge. If the aerator, stop, and line all check out and both hot and cold are weak, the cartridge is the likely cause. Shut off the stops, pull the handle and cartridge per the faucet maker's instructions, and clean or replace it. Soaking a removed cartridge in white vinegar can free up scaled passages, but a worn one is cheap to replace and worth swapping while you have it apart. Match the replacement to your faucet brand and model number, since cartridges are not interchangeable between makers. Take the old part to the store or photograph the brand stamp before you shop.
When to call a plumber
Most single-fixture pressure problems are a cleaning or a part swap. Call HQ Plumbing & Air at (602) 675-1555 if the stop valve is seized or leaks when you turn it, if a cartridge is stuck or you cannot source the right part, or if hot water alone is weak at several fixtures, which can mean sediment or a valve issue at the water heater rather than at the tap. Also call if you clean the aerator and showerhead and pressure is still poor, since a hidden line restriction or a pinhole leak behind the wall needs a pro to find. The International Plumbing Code lists minimum design pressures in Table 604.3, such as 8 psi at a flush-tank fixture and 15 psi at a flushometer valve; a fixture running well below its neighbors is worth fixing before scale spreads. For low flow tied specifically to a clogged faucet screen, see our low-water-flow-from-one-faucet-aerator page.